and they don't. >> reporter: the risk of breast cancer is 10 to 20% higher than state average, the big question is why. researchers track breast cancer neighborhood by neighborhood. and found among the state's highest risks neighborhoods ring in the bay. >> that's scary, i want to show why. >> environment, environmental immaterial pact. >> reporter: it's southern sonoma, western contra costa and alameda counties, western santa clara and san mateo county, not san francisco. but eastern marin county where there's already active research on breast cancer clusters. >> it's an opportunity to look at those areas, all those areas and see what's different and what's similar. >> reporter: barlow and researchers say they did not find a common cause. delayed childbirth, bottled water have all been proposed for the elevated risk.